1. Field
The embodiments described herein relate generally to systems for generating megavoltage radiation. More particularly, the described embodiments relate to imaging based on megavoltage radiation.
2. Description
A linear accelerator produces electrons or photons having particular energies. In one common application, a linear accelerator generates a radiation beam exhibiting megavoltage energies and directs the beam toward a target area of a patient. The beam is intended to destroy cells within the target area by causing ionizations within the cells or other radiation-induced cell damage.
Imaging systems may be used to verify patient positioning prior to the delivery of treatment radiation. According to some examples, a radiation beam is emitted by a linear accelerator prior to treatment, passes through a volume of the patient and is received by an imaging system. The imaging system produces a set of data that represents the attenuative properties of objects of the patient volume that lie between the radiation source and the imaging system.
The set of data is used to generate a two-dimensional portal image of the patient volume. The portal image will include areas of different intensities that reflect different compositions of the objects. For example, areas of low radiation intensity may represent bone and areas of high radiation intensity may represent tissue. Several two-dimensional portal images may be acquired from different perspectives with respect to the patient volume and combined to generate a three-dimensional image of the patient volume. The foregoing images may be used to diagnose illness, to plan radiation therapy, to confirm patient positioning prior to therapy, and/or to confirm a shape and intensity distribution of a radiation field prior to therapy.
Conventional imaging systems utilize thin (e.g., <1 mm) phosphor screens to convert incoming X-ray photons into light. The light is then converted into electric charge by an array of photodiodes. Such systems are efficient in converting low-energy X-rays (e.g. 0 to 250 keV) into light and therefore produce sufficiently detailed images when exposed to such X-rays. However, conventional thin screen-based systems are rather inefficient in stopping and converting megavoltage X-rays into light. As a result, conventional systems are unable to produce satisfactory images based on megavoltage radiation.
The above-mentioned stopping and converting efficiency may be improved by using thicker phosphor screens, but light spreading within such screens results in blurred images. Thick structured scintillator blocks composed of Csl needles have also been contemplated. Light transmission efficiency decreases as the thickness of such blocks increases, which leads to reduced total system sensitivity.
U.S. Pat. No. 7,030,386 describes a megavoltage radiation detector with improved quantum efficiency with respect to conventional systems. This detector is several centimeters thick and primarily composed of slabs exhibiting high electron density. Electrodes are placed within a lower density ionizable material (or a material capable of directly converting megavoltage radiation to electric charge) that is sandwiched between the slabs. In operation, an incident photon scatters in the slabs, electrons are produced by the scattering, the electrons ionize the ionizable material, and the resulting current flow is captured by an electrode located within the ionizable material. X-ray images are generated by recording these currents across a 2-D array of electrodes.
The foregoing detector presents several inefficiencies. These inefficiencies include a high manufacturing cost and poor scatter rejection. It would therefore be beneficial to provide a more suitable detector for generating satisfactory images based on incident megavoltage radiation. Such images may provide improved patient diagnosis, treatment planning and/or treatment delivery.